In a magnetic resonance imaging (“MRI”) area, linear magnetic field gradients can be used for spatial encoding. Gradient coils are generally used to produce the linear magnetic field gradients. The gradient coils can be designed to provide an imaging field-of-view that may be fixed in size. In “whole-body”applications, the gradient coil may be designed to produce sufficiently linear or uniform magnetic field gradients over a volume that is larger than e.g., the volume for a dedicated cardiac scanner. As the useful volume is decreased, the stored energy of the gradient coil is generally reduced, which may allow for a higher system performance, and particularly, provide higher peak gradient strengths and faster gradient coil switching. Externally from the substantially linear region of the gradient field (and to a lesser extent within such region) the magnetic field gradients produce image distortion. Software-based distortion correction schemes have been developed to correct for non-uniformities within the useful volume, and to somewhat expand the useful imaging field of view beyond the linear region.
The gradient coils are heavy electromechanical devices, unlike most RF surface coils, which can be easily removed and replaced with different RF coils between imaging procedures. A gradient coil, due to its high power nature and the high forces created when it is energized, is generally firmly fixed within an MRI system. As such, a dedicated gradient coil tends to make the MRI system a special-purpose imaging system, thus limiting its scope and use for clinical application. Thus, a given geometry gradient coil may result in a corresponding field of view.
MRI systems and methods use the magnetic field strength dependence of the frequency of magnetic resonance, in conjunction with computer-controlled magnetic field gradients, to produce images based on the magnetic resonance signal. The mapping of position into frequency through the gradient can be used for a reconstruction of the image from signals that are detected in the presence of the gradients or after the gradients have been applied (e.g., using “frequency” or “phase”encoding, respectively). In addition, the excitation of the magnetic resonance in the presence of the gradient can be used for a selective excitation of a “slice” or “slab” to be imaged. Other uses of the gradients in MRI systems and methods include a suppression of unwanted signals by “spoiling” excited resonance and a sensitization of the signal to particular aspects of motion, such as a bulk velocity or a diffusion.
Although conventional magnetic resonance imaging systems include magnetic field gradient-generating coils, such coils are generally designed to have very uniform magnetic field gradients (i.e., a very linear field variation with position) over a relatively large volume of the imaging system magnet bore. These coils allow for a flexibility of choice of structures to be imaged with faithful reproduction of the subject's geometry; however, there are some disadvantages associated therewith.
First, the practically achievable strength of the gradient field is limited because of the high power demands that are generally used to fill a large region with a uniform gradient. Thus the achievable image resolution and the ability to use strong gradients for other purposes, such as measuring diffusion, are limited.
Second, the achievable gradient switching speeds are also limited, because of power limitations (which are greater for a larger region of uniform gradient) and because of the physiologic limits on the allowable local rate of change of the magnetic field (which are more restrictive for larger regions of uniform magnetic field gradient). Thus the imaging speed and the ability to use the rapidly switched gradients for other purposes (such as measuring short T2 relaxation times) are limited.
Third, if a large subject is imaged with uniform gradients, a large amount of imaging data should be used to acquire a correspondingly large amount of the imaging data to reconstruct the image without artifacts in a given region of interest. This is due to aliasing of remote structures, particularly with phase encoding, even if the images of much of the rest of the region being reconstructed are not within the area of interest. If it is previously known that only a relatively restricted portion of the subject's volume is the area of interest, then the image acquisition speed can be increased because the need to cover a large region with a uniform magnetic field gradient may be removed.
In the past, attempts have been made to address these deficiencies. For example, an overview of gradient coil design was described in: Turner R., “Gradient coil design: a review of methods,” Magnetic Resonance Imaging 11: 903-920; 1993, which is incorporated herein by reference in its entirety. As described in this publication, localized gradient coils can be used for an increased resolution imaging of superficial or relatively localized structures. However, these conventional approaches have generally employed the concept of constructing a smaller gradient coil to be used inside the bore of an MRI system, with a relatively uniform gradient field in the vicinity of the coil to be used for localized imaging. Other methods have been previously described to correct for the geometric distortion that is introduced by gradient nonlinearity, which are generally available on most manufacturer's MRI systems. However, these methods currently have not been used in conjunction with the gradients designed to have fields with specific controlled nonlinearity, as addressed in further detail below in accordance with the present invention. Certain manufacturers have developed MRI systems with two sets of gradient coils mounted in the bore of the magnet, i.e., a longer conventional coil and a shorter concentric coil for higher performance imaging of shorter regions: such system is described in Harvey P R, Katznelson E., “Modular Gradient Coil: a New Concept in High-Performance Whole-Body Gradient Coil Design,” Magnetic Resonance in Medicine 42:561-570; 1999, which is incorporated herein by reference in its entirety. However, these systems are not designed to take the full advantage of the possibilities offered by reducing the need for high gradient uniformity in the imaging region. This is because the effective geometries of such coils are not electrically adjustable, as compared to the arrangement according to the present invention, other than offering the possibility of switching between using either of the coils separately or together.